1. Title of the Invention
The present invention relates to semi-oval-shaped (i.e., a shape having parallel portions and semi-circular arc portions at each end thereof) metallic carrier having an excellent thermal stress resistance and thermal fatigue resistance and used for supporting an automobile exhaust gas-purifying catalyst.
The most common type of carrier for supporting an automobile exhaust gas-purifying catalyst is formed as a honeycomb body consisting of superimposed flat metal foils (hereinafter referred to as "flat foils") and corrugated metal foils (hereinafter referred to as "corrugated foils") made of a heat resistant stainless steel, and wound together. The cross section of the carrier is usually circular, but there is a great demand for a semi-oval-shaped metallic carrier, because such a carrier is often mounted while surrounding a part of an engine.
Such metallic carriers must be able to withstand a thermal stress and thermal fatigue caused by a heat cycle dependent on heating and cooling treatments and a temperature distribution difference in a honeycomb body. When such a metallic carrier is subjected to the heat cycle, there occurs a large temperature difference between the jacket enclosing the honeycomb body and the outermost corrugated foil of the honeycomb body, and accordingly, a large thermal stress is developed on the outermost corrugated foil. Therefore, a means for suppressing this thermal stress becomes necessary, to enable the honeycomb body to be fixed to the jacket.
As described in, for example, Japanese Unexamined Utility Model Publication Nos. 61-162329 and 62-160728, a method is known of fixing a honeycomb body by folding down a jacket at the end face of the honeycomb body. In this method, the jacket and the honeycomb body are not joined together between the end faces, and thus no thermal stress is caused by a binding of the jacket.
Nevertheless, since the jacket is fixed to the honeycomb body at the end face thereof the honeycomb body is often ruptured due to the vibration of the engine or a thermal expansion elongation difference between the honeycomb body and the jacket, and if a thermal stress is repeatedly imposed on the carrier, a gap is gradually produced between the jacket and the honeycomb body, and the carrier becomes loose at that joint. Once the carrier becomes loose at that joint, the rupture of the honeycomb body becomes more and more severe, and the gap between the jacket and the honeycomb body becomes larger. As a result, the honeycomb body is vibrated and banged against the jacket, and the shock of such impacts causes the supporting slurry to fall from the catalyst, to thereby lower the purification capability of the catalyst.
Further, as disclosed in Japanese Unexamined Patent Publications Nos. 63-36842 and 62-273052, a method is known of fixing a honeycomb body by passing a pin through the honeycomb body or by fixing a plate therein. In this method, however, since the pin or plate is placed in the honeycomb body, these objects are deformed when exposed to very high temperatures, and thus lose their ability to fix the honeycomb body, and as a result, the honeycomb body is separated from the jacket, to thereby worsen the condition of the engine.
Furthermore, as disclosed in Japanese Unexamined Utility Model Publication No. 63-22319, a method is also known of fixing a honeycomb body by protuberances provided on the inner surface of a jacket. In this method, however, when a heat cycle is repeated, the honeycomb body crumbles at the protruding portions, whereby gaps are formed between each of the protuberances and the honeycomb body, and thus the fitting of the honeycomb body becomes loose.
As described above, none of the mechanically fixing methods can avoid an unstable fitting, and thus the efficiency of the metallic carrier is low.
The gist of the device described in Japanese Unexamined Utility Model Publication No. 62-194436 is a suppression of the binding between a honeycomb body and a jacket at an axial-directional open end, by joining them at a cross-sectional part of the honeycomb body, and in the examples of this publication, the jacket is joined to the honeycomb body by brazing. Although this method is useful for the suppression of stress developed in the axial direction, it is of no use for a suppression of stress developed in the radial direction, in cross section. In particular, when a radius in the direction of a major axis is long, as in a semi-oval-shaped carrier, the suppression of stress in the radial direction becomes important. In a method such as disclosed in Japanese Unexamined Utility Model Publication No. 62-194436, the thermal stress applied to a honeycomb body cannot be sufficiently suppressed, and a large thermal stress is imposed at the outermost corrugated foil of the honeycomb body, at which the honeycomb body is joined to a jacket, and thus the outermost corrugated foil is broken and becomes separated from the jacket.
U.S. Pat. No. 4,795,615 discloses a technique for joining a honeycomb body and a jacket, the object of this technique being to avoid an elongation in an axial direction of the honeycomb body from the jacket. Also, in this technique the locations of the junctions between the foils of the honeycomb body and between the honeycomb and the jacket are not duplicated in the axial directions.
Nevertheless, in a semi-oval-shaped carrier, a difference of the length of the major axis and the minor axis becomes large, and a deformation of the honeycomb body during a heat cycle is different at the major axis and at the minor axis, and thus a large thermal stress is imposed at the junction between the jacket and the honeycomb body, whereby the outermost corrugated foil thereof is broken and becomes separated from the jacket. This problem is not solved by a location of one junction between the jacket and the honeycomb body.
As described above, in the conventional techniques for fixing a jacket and honeycomb body to each other, the defects mentioned above cannot be overcome, and thus the true efficiency of a metallic carrier cannot be exhibited.